PROJECT SUMMARY The emotion of fear is a powerful trigger for memory storage, helping us avoid cues associated with aversive outcomes. However, fear memories become maladaptive and trigger inappropriate emotional responses in common psychiatric diseases such as post-traumatic stress disorder (PTSD), generalized anxiety disorder, and panic disorder. Women are twice as likely as men to experience fear-based psychiatric conditions, and greater symptom severity leads to poorer quality of life in women with these illnesses. Unfortunately, the neurobiological mechanisms underpinning this sex difference in susceptibility remain poorly understood due to a lack of female subjects in preclinical research. By understanding how females encode and express fear memories differently than males, we will pinpoint sex-specific mechanisms and treatment strategies for fear- based psychiatric disease. The purpose of this project is to identify the molecular-, cellular-, and circuit-based mechanisms underlying female susceptibility to fear-based mental illness. Based on preliminary experiments, this work will focus on the lateral septum (LS), a sexually dimorphic hub of the limbic system known to gate maladaptive emotional behaviors. We show that the LS is robustly activated during cued threat (also known as fear) memory formation in females but not males, that this sex-specific activation is causally linked to the strength of the threat memory, and that neurons of the female LS threat memory ensemble directly project to regions that drive cue-evoked emotional responses. The experiments proposed in this application will expand upon these initial findings to test the central hypothesis that female-specific engagement and subsequent synaptic strengthening of a hitherto unrecognized LS circuit enhances cued threat memory in females versus males. In Aim 1, we will test the hypothesis that selective recruitment of threat-related LS neurons enhance threat memory encoding and expression in females versus males through disinhibition of projection neurons of the hypothalamus, ventral hippocampus, and periaqueductal gray. In Aim 2, we will test the hypothesis that excitatory projections from the medial prefrontal cortex are necessary and sufficient for LS memory ensemble recruitment and enhanced memory encoding in females versus males. Finally, in Aim 3, we will test the hypothesis that sex differences in the molecular architecture of LS neurons drive sex-specific memory allocation. These experiments have the potential to transform our understanding of how basic brain circuits for fear memory storage and expression differ between the sexes. Our long-term objective of this work is to help clinicians uncover novel gender-specific therapeutic targets for fear-based psychiatric disease.